Prior Art Methods for Characterizing Forces
Three methods have been described in the prior art for determining quantities related to the position, magnitude and distribution of the forces exerted by a subject's foot (or the two feet combined) against a support surface during standing or walking.
A. Coupled Force Transducers
One method described in the prior art is the forceplate, typically a flat, rigid surface which mechanically couples three but more often four linear force transducers. FIG. 15A shows a typical prior-art forceplate 20 with four force transducers 29, and FIG. 15B shows another prior-art forceplate 20 with only three force transducers. The following examples of published studies each describe a forceplate comprised of linear force transducers coupled to a substantially rigid plate to form a single force measuring surface, and each describes methods by which the force measuring surface is used to quantify aspects of the forces exerted by the feet of a subject standing on the forceplate; Nashner, L. M. (1970) "Sensory Feedback in Human Posture Control," Massachusetts Institute of Technology Report, MVT-70-3; Black, et al. (1978) "Computerized Screening of the Human Vestibulospinal System," Annals of Otology, Rhinology and Laryngology, 87:853-861; and Hassan, et al. (1990) "Effect of Loss of Balance on Biomechanics Platform Measures of Sway: Influence of Stance and Method of Adjustment," Journal of Biomechanics, 23:783-789. U.S. Pat. No. 4,136,682, to Pedotti, describes a substantially rigid platform, a plurality of force transducing members, and a hybrid processor for analyzing the space-time representation of the resultant forces exerted by a subject walking on the rigid platform. The most commonly determined quantities used to describe the forces exerted on the forceplate surface by an external body are the following: (1) the position (in the horizontal plane) of the center of the vertical axis component of force, (2) the magnitude of the vertical axis component of the center of force, and (3) the magnitude of the two horizontal axis components (anteroposterior and lateral) of the center of force. Calculation of position and magnitude quantities for the vertical axis component of the center of force requires that only the vertical force component be measured by each of the three (or four) mechanically coupled force transducers. To measure the horizontal axis components of force, the force transducers must also measure the horizontal plane components of force.
The exact form of the calculations required to determine the above described center of force position and magnitude quantities from the measurement signals of the linear force transducers depends on the number and positions of the force transducers. Specifically, these algorithms must take into account the known distances between the force measuring transducers. Algorithms for performing such calculations are well documented in the prior art. The Operators Manual for the EquiTest system manufactured by NeuroCom International, Inc. of Clackamus, Oreg., the assignee herein, for example, describes mathematical formulae for calculating the position (anteroposterior and lateral components) and the magnitude of the vertical axis component of the center of force exerted on a mechanically coupled 4-transducer forceplate. Also described are the calculations necessary to determine the magnitude of the horizontal axis (anteroposterior component) of the center of force using an additional linear force transducer with sensitive axis in the anteroposterior horizontal axis of the forceplate. A more complex forceplate manufactured by Advanced Mechanical Technologies, Inc. of Newton, Mass., uses mechanically coupled multi-axis force transducers to measure all of the vertical axis, anteroposterior horizontal axis, and lateral horizontal axis force components.
When a forceplate is used to measure quantities related to the position of the center of force, the position quantity is always determined in relation to coordinates of the forceplate surface. If the position of the foot exerting the force on the surface is not precisely known in relation to the forceplate surface, or if the position of the foot changes with time relative to the surface, the position of the center of vertical force cannot be determined in relation to a specified anatomical feature of the foot.
B. Instrumented Shoe
A second method described in the prior art for measuring quantities related to forces exerted by a foot against a supporting surface during standing and walking is a shoe in which the sole is instrumented with linear force transducers. One example of a system incorporating force measuring transducers in a shoe is the Computer Dyno Graph (CDG) System manufactured by Infotronic Medical Engineering of Tubbergen, The Netherlands. Other examples of systems incorporating force measuring transducers into a shoe was described in Spolek, et al. (1976) "An Instrumented Shoe. A Portable Force Measuring Device," Journal of Biomechanics 9:779-783 and Ranu, H. S. (1987) "Normal and Pathological Human Gait Analysis Using Miniature Triaxcial Shoe-Borne Load Cells," American Journal of Physical Medicine 66:1-10. The principles for determining the position of the center of vertical force exerted on the sole of the shoe by the subject's foot are mathematically similar to those used to calculate the position of the center of force quantities using the forceplate.
Because the position of an instrumented shoe is fixed in relation to the foot, the instrumented shoe can be used to determine the position of the center of vertical force in relation to coordinates of the foot, regardless of the position of the foot on the support surface. A disadvantage of the instrumented shoe is that the position of the center of vertical force cannot be determined in relation to the fixed support surface whenever the position of the foot on the support surface changes during the measurement process. Another disadvantage in a clinical environment is that the subject must be fitted with an instrumented shoe.
C. Independent Force Transducers
A fundamentally different method described in the prior art for determining quantities related to the forces exerted on a support surface utilizes a plurality of mechanically independent vertical force transducers. Each vertical force transducer measures the total vertical force exerted over a small sensing area. The independent transducers are arranged in a matrix to form a force sensing surface. The two-dimensional position in the horizontal plane and the magnitude of the vertical component of the center of force exerted on the sensing surface can be determined from the combined inputs of the mechanically independent transducers. When the vertical force transducers are not mechanically coupled, however, the accuracy of the center of vertical force position quantity will be lower, and depends on the sensitive area of each transducer and on the total number and arrangement of the transducers. When mechanically independent vertical force transducers are used to determine the position of the center of vertical force, the resulting quantities are determined in relation to coordinates of the force sensing surface. Two examples of systems which use grids of independent force or pressure measuring transducers to map the distribution of forces during human gait analysis are the F-Scan system described in an article by Podoloff, R. M. (1991) "A Pressure Mapping System for Gait Analysis," Sensors, May, 1991, pp. 21-25 and the Peruchon, et al. (1990) "Individual Gait Characterization from Dynamic Analysis of Plantar Force Distribution" in: Brandt, et al. (eds.) Disorders of Posture and Gait, George Thieme Verlag, N.Y., pp. 62-66.
The plurality of independent force measuring transducers can be used to determine additional quantities related to the distribution of forces exerted against a support surface by a subject's foot. FIG. 5 in the Podoloff article and FIG. 3 in the Peruchon, et al. article show examples of the outlines that can be produced by a system for mapping the distribution of pressures exerted by the foot on the surface. As described in the articles, the positions of anatomical features of the foot such as the heel, the ball, and the toes can be identified from the foot pressure maps. When the position of a first anatomical feature is determined in relation to the support surface by the pressure mapping means, the position of a second anatomical feature of the foot can be determined in relation to the support surface by the following procedure. The linear distance between the first and second anatomical features is determined. Then, the position of the second anatomical features in relation to the support surface is determined to be the position of the first anatomical feature in relation to the support surface plus the linear distance between the first and second anatomical features.
Measurement of Support Surface Reaction Forces
When a subject stands with a foot placed in a fixed position on the surface of a force sensing surface, the position of the center of force exerted by the foot can be determined in relation to coordinates of the forceplate surface. If the position of a specified anatomical feature of the foot (for example, the ankle joint) is also known in relation to the coordinates of the forceplate surface, the position of the center of force in relation to coordinates of the specified anatomical feature of the foot can be determined by a coordinate transformation in which the difference between the force and anatomical feature position quantities are calculated.
Analysis of Support Surface Reaction Forces During Gait
Forceplates, instrumented shoes and independent force transducers have all been used in the prior art to measure quantities related to the position and magnitude of the center of force exerted by each foot against the support surface during stepping-in-place, walking, and running. Forceplates embedded in walkways have measured quantities related to the position and magnitude in relation to the fixed (forceplate) support surface for single strides during over-ground walking and running (for examples; Nashner, L. M. (1980) "Balance Adjustments of Humans Perturbed While Walking," J. Neurophysiol. 44:650-664; Andriacchi, et al. (1977) "Walking Speed as a Basis for Normal and Abnormal Gait Measurements," Journal of Biomechanics 10:261-268; and Winter, D. A. (1980) "Overall Principle of Lower Limb Support During Stance Phase of Gait," Journal of Biomechanics 13:923-927). Using additional information on the position of a specified anatomical feature of the foot in relation to the forceplate support surface, the position of the center of force has also been determined in relation to a specified anatomical feature of the foot.
The position and the magnitude of the center of force exerted by a foot against the support surface have been determined relative to anatomical features of the foot by embedding force transducers in the shoes of walking and running subjects (for examples, Ranu, Podoloff and Spolek). Measures of the timing of heel-strikes and toe-offs have been made using contact switches embedded in the subject's shoes (Ishida, et al. (1990) "Evaluation on the Stepping Movement with a Recording of Plantar Switching" in: Brandt, et al. (eds.) Disorders of Posture and Gait, George Thieme Verlag, New York, pp. 58-61).
Categories of Gait
The phases of human gait have been described by many authors; for examples, Inman, et al. (1981) "Human Walking," Williams and Wilkins, Baltimore; Winter, D. A. (1983) "Biomechanical Motor Patterns in Normal Walking," Journal of Motor Behavior 15:302-330; and Winestein, et al. (1989) "Quantitative Dynamics of Disordered Human Locomotion: a Preliminary Investigation," Journal of Motor Behavior 21:373-391.) Human gait may be classified in general categories of walking and running. During walking, at least one foot is always in contact with the support surface and there are measurable periods of time greater than zero during which both feet are in contact with the support surface. During running, there are measurable periods greater than zero during which time neither foot is in contact with the support surface and there are no times during which both feet are in contact with the support surface.
Walking can be separated into four phases, double support with left leg leading, left leg single support, double support with right leg leading, and right leg single support. Transitions between the four phases are marked by what are generally termed "heel-strike" and "toe-off" events. The point of first contact of a foot is termed a "heel-strike" because in normal adult individuals the heel of the foot (the rearmost portion of the sole when shoes are worn) is usually the first to contact the surface. However, heel-strike may be achieved with other portions of the foot contacting the surface first. During running normal adult individuals sometimes contact with the ball of the foot (forward portions of the sole when shoes are worn). Individuals with orthopedic and/or neuromuscular disorders may always contact the surface with other portions of the foot or other points along the perimeter of the sole when shoes are worn. Similarly, while the ball and toes of the foot are the last to contact the surface at a toe-off event in normal adults, a patient's last point of contact may be another portion foot. Thus, regardless of the actual points of contact, the terms heel-strike and toe-off refer to those points in time at which the foot first contacts the support surface and ceases to contact surface, respectively.
Characterization of Gait Using a Treadmill
Treadmills allowing a subject to locomote over a range of walking and running speeds within a confined space have been described in the prior art (Traves, et al. (1983) "A Speed-Related Kinematic Analysis of Overground and Treadmill Walking" in: Winter, et al. (eds.) Biomechanics XI, Human Kinetics Publishers, Champaign, pp. 423-426; Nelson, et al. (1972) "Biomechanics of Overground Versus Treadmill Running," Medicine and Science in Sports 4:233-240; and Charteris, et al. (1978) "The Process of Habitation to Treadmill Walking: a Kinetic Analysis," Perceptual and Motor Skills 47:659-666). A treadmill allows the difficulty of gait to be precisely set by independently controlling the belt speed and the inclination of the belt. The subject can be maintained in a fixed position relative to the measuring surface underlying the treadmill belt by coordinating the speed of gait with the speed of the treadmill belt movement.
Several prior art research studies have described treadmills in which a single forceplate with mechanically coupled force transducers has been mounted directly beneath the treadmill belt. Kram et al., in their paper "A Treadmill-Mounted Force Platform", Journal of The American Physiological Society, 1989, pages 1692-1698, describe a treadmill having a single forceplate. This paper is enclosed herewith and hereby incorporated herein by reference. The single forceplate provided continuous measurement of the forces exerted by the combined actions of the two feet on the overlying treadmill belt during gait.
It is sometimes desirable to determine the position of the center of force in relation to coordinates of specified anatomical features of the foot when the foot is in contact with a surface which is moving in relation to a fixed force sensing surface. This occurs, for example, when the foot is contacting the moving belt of a treadmill which overlays a force sensing surface. To determine the position of the center of force in relation to coordinates of the specified anatomical features of the foot, two coordinate transformations are performed. One, the position of the center of force is determined in relation to coordinates of the moving treadmill belt. Two, the position of the moving treadmill belt is determined in relation to coordinates of the specified anatomical feature of the foot. To perform the first of these coordinate transformations requires knowledge of the treadmill belt position in relation to the fixed force sensing surface position on a continuous basis. To perform the second of these two coordinate transformations requires knowledge of the position of the specified anatomical features of the foot in relation to the treadmill belt. Since the position of the foot and its anatomical features does not change in relation to the treadmill belt following each heel-strike event and before the subsequent toe-off of that foot, the position of the specified anatomical features of the foot needs be determined only once at heel-strike for each step.
One method to determine the position of the treadmill belt on a continuous basis in relation to the fixed force sensing surface is to use one of several sophisticated commercial treadmill systems described in the prior art which measure the anteroposterior speed of the moving treadmill belt on a continuous basis, and which provide the means to regulate the belt anteroposterior speed on a continuous basis. One example of a commercially available treadmill system with automatic speed control and belt speed measurement systems is the Star Trac 2000, manufactured by Unisen, Inc., Tustin, Calif. When one of these treadmill systems is used, the information necessary to determine the continuous position of the treadmill belt in relation to the underlying forceplate is obtained by performing mathematical integration of the belt speed signal on a continuous basis.
There are methods described in the prior art which can be used to determine, at the time of heel-strike, the position of the moving treadmill belt in relation to the specified anatomical features of the foot. One method is to use one of several commercially available optical motion analysis system. Two examples of commercially available motion analysis systems which describe applications for tracking the motions of identified points on the human body during locomotion include the ExpertVision system manufactured by MotionAnalysis Corp., Santa Rosa, Calif. and the Vicon system manufactured by Oxford Medilog Systems, Limited, Oxfordshire, England. In accordance with this method, one or more optical markers are placed on the specified anatomical features of the foot. One or more additional markers are placed on the treadmill belt at predetermined positions. The number and placement of the optical markers on the anatomical feature and the treadmill belt determine the accuracy of the measurement as specified by the systems manufacturers. At the time of heel-strike, the positions of the treadmill belt marker or markers are then determined in relation to the positions of the anatomical feature marker or markers in accordance with methods specified by the system manufacturer.
The prior art has not described devices and methods for separately determining quantities related to the force exerted by each foot against the treadmill belt support surface at all phases of the step cycle, nor has the prior art described a means for determining quantities related to the position of the center of force exerted by each foot in relation to a fixed point on the treadmill belt or in relation to a specified anatomical feature of the foot or to train normals to alter their gait patterns.